The present invention relates to a method and apparatus for supervising a digital protective relaying system wherein electrical quantities in an electric power system are detected, digitized and operated on for use in protection of the electric power system.
FIG. 1 shows an example of conventional supervising apparatus for use in a protective relaying system. An electrical quantity, such as a voltage, in an electric power system is applied to the primary of the transformer 11 and is converted to a voltage signal V.sub.1 '. Normally, a contact 12.sub.a of an auxiliary relay is open and a contact 12.sub.b of the auxiliary relay is closed, and the voltage signal V.sub.1 ' is fed to a judging circuit 13 where judgement for protection is made. To check whether there is a fault in the judging circuit 13, the contact 12.sub.b is opened and the contact 12.sub.a is closed, and a test voltage signal V.sub.2 supplied from a special power source or derived from the power system is applied to the judging circuit. By varying the magnitude of the test voltage signal V.sub.2, one can check at which level the judging circuit operates.
Recently, power systems have become huge and complicated, so that protective relaying systems are required to have improved reliability in order to maintain the stability of the power system. Digital computers are now in use for the purposes of improving the performance, the efficiency and the reliability of the control and the measurement of the power system. In view of the above, digital protective relaying systems have been developed wherein electrical quantities in the power system are digitized, and the data coded in digital form are processed in a mini-computer or a micro-computer to perform digital judgement. In general, the phase currents, the zero-phase current, the phase voltages and the zero-phase voltage are all introduced into the computer in order that a variety of protective functions are afforded by the protective relaying system. However, to introduce all the electrical quantities listed above, the device, like the one shown in FIG. 1, for applying a test voltage signal needs to be provided for each of the detected electrical quantities. As a result, the construction becomes complicated.